Graphon

Fig. XI-10. Isotherm of composition change or surface excess isotherm for the adsorption of (1) benzene and (2) n-heptane on Graphon. (From Ref. 141.)...

As a quite different and more fundamental approach, the isotherms of Fig. XI-10 allowed a calculation of X as a function of temperature. The plot of In K versus 1 /T gave an enthalpy quantity that should be just the difference between the heats of immersion of the Graphon in benzene and in n-heptane, or 2.6 x 10 cal/m [141]. The experimental heat of immersion difference is 2.4 x 10 cal/m, or probably indistinguishable. The... 

Fig. XVn-21. (a) Differential heat of adsorption of N2 on Graphon, except for Oand , which were determined calorimetrically. (From Ref. 89.) (b) Differential heat of adsorption of N2 on carbon black (Spheron 6) at 78.5 K (From Ref. 124).

Fig. XVII-23. (a) Entropy enthalpy, and free energy of adsorption relative to the liquid state of N2 on Graphon at 78.3 K (From Ref. 89.) b) Differential entropies of adsorption of n-hexane on (1) 1700°C heat-treated Spheron 6, (2) 2800°C heat-treated, (3) 3000°C heat-treated, and (4) Sterling MT-1, 3100°C heat-treated. (From Ref 18.)...

When plotted according to the linear form of the BET equation, data for the adsorption of N2 on Graphon at 77 K give an intercept of 0.004 and a slope of 1.7 (both in cubic centimeters STP per gram). Calculate E assuming a molecular area of 16 for N2. Calculate also the heat of adsorption for the first layer (the heat of condensation of N2 is 1.3 kcal/mol). Would your answer for Vm be much different if the intercept were taken to be zero (and the slope the same) Comment briefly on the practical significance of your conclusion. 

Fig. 2.16 The molar entropy for nitrogen adsorbed ongraphitizedcarbon (Graphon) at — 189-3°C, as a function of the amount adsorbed s, = molar entropy of adsorbed nitrogen s, = molar entropy of liquid nitrogen. (Courtesy Hill, Emmett and Joyner.)...

Fig. 2.25 The differential heat of adsorption of argon on carbon blacks at 78 K, before and after graphitizalion.. Spheron O, Graphon. , and El denote molar heat of sublimation and of evaporation respectively.

Fig. 5.12 (a) Water adsorption isotherms at 20°C on Graphon activated to 24-9 % burn-off, where its active surface was covered to varying extents by oxygen complex. (b) The results of (a) plotted as amount adsorbed per of active surface area (left-hand scale) and also as number of molecules of water per atom of chemisorbed oxygen (right-hand scale). (After Walker.)... 

Fig. 4.2 Equilibrium adsorption of sodium n-dodecyl sulfate on carbon black, Ti02, and Graphon at room temperature [41].

Fig. 6 Adsorption of alkyl ether sulfates on graphon (plateau values) at 25°C (product purity 98-99.5%)...

A second widely used class of adsorbates is that of dyes. Methods using these are appealing because of the ease with which analysis may be made colorimetrically. The adsorption generally follows the Langmuir equation. Graham found an apparent molecular area of 19.7 A2 for methane blue on Graphon or larger than the actual... 

Figure Ic differs markedly from those obtained for the immersion of polar solids in water initially the heat values are small but increase with increasing amounts of preadsorbed water. Thus far, only one such curve has been reported in the literature for the system Graphon-water 90). Graphon is a graphitized carbon black which has an essentially homogeneous, homopolar surface 21). Nevertheless, a small fraction of heterogeneous sites is responsible for the limited adsorption of water on the surface of this solid. Similar curves can be expected for other hydrophobic solids.

The very low water adsorption by Graphon precludes reliable calculations of thermodynamic quantities from isotherms at two temperatures. By combining one adsorption isotherm with measurements of the heats of immersion, however, it is possible to calculate both the isosteric heat and entropy change on adsorption with Equations (9) and (10). If the surface is assumed to be unperturbed by the adsorption, the absolute entropy of the water in the adsorbed state can be calculated. The isosteric heat values are much less than the heat of liquefaction with a minimum of 6 kcal./mole near the B.E.T. the entropy values are much greater than for liquid water. The formation of a two-dimensional gaseous film could account for the high entropy and low heat values, but the total evidence 22) indicates that water molecules adsorb on isolated sites (1 in 1,500), so that patch-wise adsorption takes place. 

Heats of immersion of two solids with widely different surface characteristics 39) are given in Table IV. Rutile (TiOz) is a heteropolar hydrophilic solid, and Graphon is homopolar and hydrophobic. A homologous series of alcohols and hydrocarbons and several n-butyl derivatives are the wetting liquids. Water is included for comparison. 

The heat values are markedly higher for the polar solid immersed in polar liquids they also vary considerably with the functional group of the liquid. For Graphon, however, the heats are almost unaffected by the structural features of the wetting liquid. This nonpolar solid, despite the presence of a small amount of hydrophilic sites on its surface 0), interacts with the liquids primarily through London dispersion forces. Because of the additive nature of these forces, each adsorbed molecule tends to lie flat on such a surface 40). In the case of a polar molecule the functional group is oriented somewhat away from the nonpolar surface toward the liquid. 

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